Associate Professor Nathan Bartlett, of HMRI and the University of Newcastle, in NSW's Hunter Valley, said the reopening of the state was coinciding with the waning immunity of our most vulnerable.
But he was pleased to see the Australian Government's stance on boosters become "much clearer" after data from Israel and the UK revealed the protection afforded by vaccines had begun to wane after six months.
"Those who were vaccinated first - the elderly and those at highest risk of exposure - will now have the most amount of immunity waning," Associate Professor Bartlett said.
"It is clear the elderly are going to be vulnerable again as we open up and transmission goes up.
"It is likely to mean that those at most risk are going to be exposed just as they are becoming less protected."
Federal Health Minister Greg Hunt expects third doses of the vaccine will be rolled out to the aged care sector within weeks.
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He has flagged the remainder of the population would become eligible for a booster six months after their second jab, pending approval from the Therapeutic Goods Administration (TGA).
The TGA's decision on whether the Pfizer vaccine can be used as a booster dose is expected by next week. These third shots could be available to the general public by December.
"What is happening in the UK and Israel has really prompted the Australian government to decide we are all going to get boosted at six months," Associate Professor Bartlett said. "It has become clear that after six months there is a substantial level of waning immunity, and that allows those breakthrough infections to creep up.
"We have been seeing case numbers in the UK climbing from 30,000, to 40,000, and now they are pushing 50,000 a week. Still, the hospitalisation rate is still substantially lower this time around in the UK than it was in the previous wave, showing that those vaccines are working; that if you do get infected, you are not getting as sick."
But death rates were "creeping up" as well, he said, prompting the government to act quickly on boosters.
"The government doesn't want to get caught dawdling again this time," Associate Professor Bartlett said.
"We know the elderly don't respond as well as younger people to vaccines anyway, and that they are the ones most vulnerable to severe disease.
"It would be a disaster for our elderly - with their less optimal protection having waned the most - to be exposed to the virus. You can see why it's something we need to be concerned about, and why the government has moved very swiftly on the booster program. Vaccine supply is no longer an issue."
The severely immunocompromised are already eligible for a third shot of a COVID-19 vaccine.
"The term 'booster' assumes you have some level of immunity and you are topping it up, but really, for the immunocompromised, two shots doesn't really give them much immunity," he said. "For them, three shots is about generating some level of immunity that approaches what the rest of us have with two jabs."
Associate Professor Bartlett said Australia had again had the benefit of seeing what was happening around the world, watching the trajectory of COVID, and the impact of breakthrough infections.
"There are a lot of vaccines that require more than two jabs to give you the optimal level of protection," he said.
"We will start getting these boosters and see how that goes, and it could be that that will be enough. The intention then is to maintain as high a level of immunity as we can and ensure we are filling those gaps which are appearing in the protection of other countries."
Australia's first-dose vaccination rate is sitting at about 86 per cent, with just over 70 per cent of the eligible population double-dosed.
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